The invention relates to a flexible piston rod for an injection device, which piston rod has the form of a helical spring for moving a piston forward inside a cartridge containing a liquid. The flexible piston rod transmits a movement from a piston rod drive to a longitudinal displacement of the piston; the flexible piston rod has a distal end abutting to the piston inside the cartridge, and a proximal end being deflected away from the axis of the cartridge. The piston rod drive drives the flexible piston rod and comprises of a rotating nut element engaging the flexible piston rod.
Such flexible rods are E.g. used in the new generation of very short injection devices for distributing medicine into the body. Some medication, such as insulin, is often self-administered. The typical diabetes patients will require injections of insulin several times during the course of the day. The amount of doses, and the size of each dose is usually prescribed for the individual patient, and it is a key issue for the health condition of the patients that the injection device is able to administer a very precise dose
WO 95/09021 shows a displacement system for controlled infusion of a liquid. The flexible piston rod shown has in a preferred embodiment the form of a helical spring with narrowly adjacent turns of windings, which windings provides an external thread. A piston rod drive is provided, which transmit a drive force to the piston through a nut element having an internal thread engaging the external thread of the flexible rod. By rotating the nut element, which is locked for axial movement, the flexible piston rod will drive the piston forward inside the cartridge.
The relationship between the revolutions of the nut element and the piston advancement has to be very precise in order to deliver the correct dose of liquid through the outlet of the cartridge. With the displacement system described above, the flexible piston rod has a tendency to rotate when the nut is rotated making the displacement of the piston somewhat uncontrolable.
It is an object of the invention to provide a flexible piston rod of the above mentioned type, which will remain non rotational when the nut element of the piston rod drive is rotated, making the relation between the revolutions of the nut element and the advancement of the piston of a constant size.
This is obtained by a flexible piston rod, having the form of a helical spring made up from a number of narrowly adjacent turns of windings providing an external thread, for moving a piston forward inside a cartridge containing a liquid, said flexible piston rod transmitting a movement from a piston rod drive to a longitudinal displacement of said piston, said flexible piston rod having a distal part abutting said piston inside said cartridge and a proximal part being deflected away from the axis of said cartridge, said piston rod drive including a nut element having an internal thread engaging said external thread of said flexible piston rod and which nut element is locked against longitudinal displacement and forcedly rotated,
Which flexible piston rod is characterized in that said flexible piston rod has means locking said flexible piston rod against rotation.
The nut element has an internal thread corresponding to the external thread on the flexible piston rod. With the nut element locked against axial displacement, the rotation of the nut will advance the flexible piston rod moving the piston forward inside the cartridge. To rotate the nut element a large variety of different mechanical solutions can be used. A solution where an injection button worked by the fingers of the user drives the nut element would be very useful. It is however found most useful to use an electric motor, which through a suitable gearing can rotate the nut element.
In order to prevent the flexible piston rod from rotating when the nut element is rotated it is necessary to lock the flexible piston rod against rotation. This can according to the invention be done by guiding a protrusion on the spring in a longitudinal slot, which slot could be placed in the curved guiding path, or by making the spring noncircular and fitting the spring through a non circular guiding element.
In one preferred embodiment of the flexible piston rod according to the invention the means locking the flexible piston rod against rotation is a longitudinal spine situated at the peripheral area of the helical spring forming the flexible piston rod. If the flexible piston rod forming a helical spring has such a spine, the rod will tend to fall into a predetermined position when ever bended. If the helical spring is deflected to follow an arc of a circle, the predetermined position will be one where the spine is situated on the inwardly pointing peripheral side of the bended spring. When rotating the nut element, the flexible piston rod will remain locked in this position, and the relation between the revolutions of the nut element and the advancement of the piston will be of a constant size depending purely on the pitch of the thread connection.
In another embodiment of the flexible rod according to the invention the helical spring has an initial tension and the spine is a removal or a reduction of the initial tension in a longitudinal area of the helical spring. Removing or reducing the build-in initial tension of the helical spring forming the flexible piston rod can introduce a displaced spine into the spring without physically changing the circular appearance of the spring. The initial tension is removed or reduced in a limited angular area of each winding, which areas together forms a straight longitudinal spinal area.
In yet another embodiment of the flexible piston rod according to the invention the spine is a depression in the out-turned peripheral area of each winding covering an angular part being substantial smaller than 360 degrees. This depression making the spring non-circular is according to an appropriate embodiment of the invention introduced into the spring by applying a momentarily force or a pressure over an area on the out-turned peripheral area each winding.
In yet a preferred embodiment of the flexible piston rod according to the invention the momentarily force or the pressure is applied by forcing said flexible piston rod over a relatively sharp edge. This forced a part of the pre-stressed windings back into a non-stressed position, thereby introducing a dislocated spine into the spring.
In another embodiment of the flexible piston rod according to the invention the spine is introduced into the helical spring by physical removing a part of the out-turned peripheral area of each winding covering an angular part being substantial smaller than 360 degrees. The removing of the material forming the part of the spring to be removed is in a preferred embodiment of the flexible piston rod according to the invention done by grinding. By grinding a part of each winding of the helical spring away, the spring is given a noncircular appearance, with the chord of each winding together forming the longitudinal spine.
In a last embodiment of the flexible piston rod according to the invention the spine is introduced into the helical spring when the spring is being coiled. When manufacturing a spring the windings are being coiled into a circular shape. If however they windings, and the spring, is given a different shape, a displaced spine can be introduced into the helical spring at this point.